1. Field of the Invention
The present invention relates to a film bulk acoustic resonator and a filter, in particular, relates to a film bulk acoustic resonator and a filter that include a cavity below a resonance region in which an upper electrode and a lower electrode are facing each other through a piezoelectric film.
2. Description of the Related Art
There is increased a demand for a small and lightweight resonator and a filter with the resonator, because of a rapid diffusion of a wireless device such as a mobile phone. A dielectric and a surface acoustic wave (SAW) filter are mainly used until now. Recently, there is noted a piezoelectric film resonator and a filter including the piezoelectric film resonator that have a good characteristic at a high frequency in particular and can be downsized and monolithically-integrated.
A resonator of FBAR (Film Bulk Acoustic Resonator) type is known as one type of a piezoelectric thin film resonator. The FBAR has a laminate structure composed of an upper electrode, a piezoelectric film, and a lower electrode on a substrate. The FBAR has a via-hole or a cavity (a void) under a region of the lower electrode facing the upper electrode in order to restrain a dissipation of vibration energy to a substrate. There may be formed a void under the lower electrode through a dielectric film. The via-hole is formed if a back face of a Si substrate used as an element substrate is subjected to an etching treatment. The cavity is formed, if a resonance element such as a composite film is formed on a sacrifice layer pattern on a surface of a substrate, and the sacrifice layer pattern is eliminated at last. A film bulk acoustic resonator including a via-hole and a cavity as a void is hereinafter referred to as a via-hole type or a cavity type.
An acoustic wave excited with an inverse piezoelectric effect or an acoustic wave generated by a distortion caused by a piezoelectric effect is generated in the piezoelectric film between the upper electrode and the lower electrode, when an electrical signal at high frequency is provided to between the upper electrode and the lower electrode. These acoustic waves are converted into an electrical signal. The acoustic wave is converted into a longitudinal vibration wave having a main displacement in a thickness direction, because the acoustic wave is totally reflected at a face of the upper electrode and a face of the lower electrode that contact to air. In the element structure, a resonation is occurred at a frequency where a thickness H of a laminate structure composed of the upper electrode, the piezoelectric film and the lower electrode formed above the cavity is integral multiple of a half of the acoustic wave. A propagation velocity V of the acoustic wave is determined according to a material. A resonance frequency F is obtained by F=nV/2H (“n” is a given value). It is possible to control the resonance frequency with use of the thickness as a parameter, if the resonance is used. And it is possible to fabricate a resonator and a filter using a desirable frequency characteristic.
The upper electrode and the lower electrode may be made of a metal such as aluminum (Al), copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), ruthenium (Ru), rhodium (Rh), iridium (Ir), chromium (Cr), or titanium (Ti), or a lamination material made of the above-mentioned metal. The piezoelectric film may be made of aluminum nitride (AlN), zinc oxide (ZnO), lead zirconate titanate (PZT), lead titanate (PbTiO3) or the like. In particular, aluminum nitride and zinc oxide are suitable for the piezoelectric film because aluminum nitride and zinc oxide have an aligned axis in (002) direction when being formed. The substrate may be made of silicon (Si), glass, gallium arsenic (GaAs) or the like.
FIG. 1 illustrates a cross sectional view showing a via-hole type of a film bulk acoustic resonator disclosed in Electron. Lett., pages 507 to 509, volume 17, 1981. As shown in FIG. 1, the film bulk acoustic resonator has a laminate structure in which there are laminated an Au—Cr film acting as a lower electrode 13, a ZnO film acting as a piezoelectric film 14 and an Al film acting as an upper electrode 15 on a (100) Si substrate 11 having a thermally oxidized film (SiO2) 12. There is formed a cavity (via hole) 16 under the laminate structure. The cavity 16 is formed from a back face of the (100) Si substrate 11 with an anisotropic etching using IKOH aqueous solution or EDP aqueous solution (a liquid mixture of ethylenediamine, pyrocatechol and water).
FIG. 2 illustrates a cross sectional view showing a cavity type of a film bulk acoustic resonator disclosed in Japanese Patent Application Publication No. 60-189307. As shown in FIG. 2, the film bulk acoustic resonator has a laminate structure in which there are laminated a lower electrode 23, a piezoelectric film 24 and an upper electrode 25 on a substrate 21 having a thermally oxidized film (SiO2) 22. There is formed a cavity 26 under the laminate structure. The cavity 26 is formed with processes of forming an island shaped ZnO sacrifice layer pattern on the substrate 21 in advance, forming the laminate structure on the sacrifice layer pattern, and removing the sacrifice layer under the laminate structure with an etching liquid such as an acid.
These film bulk acoustic resonators have a resonance region in which the lower electrodes 13 and 23 and the upper electrodes 15 and 25 are facing each other through the piezoelectric films 14 and 24. A high quality factor Q is obtained when vibration energy is confined in the resonance region. For example, Japanese Patent Application Publication No. 2002-223144 (hereinafter referred to as Document 1) discloses an art where an energy loss caused by an acoustic wave propagating in a lateral direction is reduced and the quality factor Q is improved.
FIG. 3 illustrates a cross sectional view of the film bulk acoustic resonator disclosed in Document 1. A lower electrode 33 is formed on a substrate 31 through a cavity 36. A piezoelectric film 34 is formed on the lower electrode 33. An upper electrode 35 is formed on the piezoelectric film 34. A resonance region 50 is a region where the lower electrode 33 faces the upper electrode 35 through the piezoelectric film 34. A support region 52 is provided around the resonance region 50. An adjacent region 54 is provided around the support region 52. The support region 52 is composed of the lower electrode 33 and the piezoelectric film 34 above the cavity 36. The adjacent region 54 is composed of the substrate 31, the lower electrode 33 and the piezoelectric film 34. A width L of the support region 52 and the adjacent region 54 is determined to be ¼ of a wavelength of an acoustic wave propagating in a lateral direction. In this case, it is possible to confine the acoustic wave in the resonance region 50. And it is possible to improve a quality factor Q.
However, in the resonator disclosed in Document 1, a spurious (an unnecessary response) is occurred at a frequency lower than the resonance frequency. FIG. 4 illustrates a Smith chart of a resonator where a spurious is occurred. The spurious is occurred in a frequency range lower than a resonance point. A description will be given of a case where a resonator in which the spurious is occurred is used in a ladder type of a filter shown in FIG. 5. As shown in FIG. 5, series resonators S1 through Sn are connected in series between an input terminal In and an output terminal Out. Parallel resonators P1 through Pn are connected in parallel between a ground and the input terminal In and the output terminal Out. FIG. 6 illustrates an amount of a loss with respect to a frequency in a case where the resonator in which the spurious is occurred is used in the ladder type of the filter. A characteristic degradation caused by the spurious is observed in a frequency range A on the lower frequency side in a transmitting property. It is important to restrain the spurious in the resonator.